MainWindow.h

First we Subclass KMainWindow on line 7 with with class MainWindow : public KMainWindow.

The we declare the constructor with MainWindow(QWidget *parent=0);.

And finally we declare a ponter to the object that will make up the bulk of our program. KTextEdit is a generic richtext editor with some KDE niceties like cursor auto-hiding.

MainWindow.cpp

include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent) : KMainWindow(parent)
{

textArea = new KTextEdit;
setCentralWidget(textArea);
setupGUI();

}
First, of course, on line 1 we have to include the header file containing the class declaration.

On line 5, we initialise our text editor with an object. Then on line 6 we use the built-in setCentralWidget() function which tells the KMainWindow what should appear in the central section of the window.

Finally, KMainWindow::setupGUI() is called which does a lot of behind the scene's stuff and creates the default menu bars (Settings, Help).

Back to main.cpp

In order to actually run this window, we need to add a few lines in main.cpp:

}
The only new lines here (compared to Tutorial 1) are 16 and 17. On line 16, we create our MainWindow object and then on line 17, we display it.

CMake

The best way to build the program is to use CMake. All that's changed since tutorial 1 is mainwindow.cpp has been added to the sources list.
project (tutorial2)
find_package(KDE4 REQUIRED)
include_directories( ${KDE4_INCLUDES} )
set(tutorial2_SRCS
main.cpp
mainwindow.cpp
)
kde4_add_executable(tutorial2 ${tutorial2_SRCS})
target_link_libraries( tutorial2 ${KDE4_KDEUI_LIBS})